Due to excellent electrical insulation, heat resistance, weather resistance and flame retardance, silicone rubber is used in a wide variety of applications including electric and electronic equipment (such as domestic appliances and computers), transportation vehicles, business machines, and buildings. Particularly in the latest decade, silicone rubber is used as coatings on heat sinks in computers and fixing rolls like heating rolls and pressure rolls in copiers and laser printers. To comply with higher speed copiers and color copiers which are recently on wide-spreading use, fixing rolls are required to have lower hardness. Prior art metal materials and fluororesins fail to meet such requirements, and rolls of the type in which fluororesin is coated over heat conductive silicone rubber are often employed. In particular, the rubber used in heating rolls is required to have a high thermal conductivity from the standpoints of reducing the waiting time upon startup of the machine and saving the energy consumed by the machine itself. A low compression set is also required because the rubber is always exposed to high temperatures of 150 to 250° C. However, since silicone rubber itself is not so heat conductive, fillers having a high thermal conductivity are generally added. Such filled silicone rubber compositions are disclosed, for example, in JP-A 58-219259 corresponding to U.S. Pat. No. 4,444,944, JP-A 3-221982 and JP-A 10-39666. In these compositions, silica, alumina, magnesium oxide and the like are added to ordinary silicone rubbers as the heat conductive filler. These fillers, however, must be loaded in large amounts in order to improve heat conductivity. Heavy loading of fillers adversely affects the compression set of rubber which is requisite as rubber rollers, and gives rise to detrimental problems including degraded heat resistance, increased roll hardness, and difficulty of molding.